Can't decide so...
(Do not feel like explaining the abbreviations)
C.G.-R2 OP 2
G.C.-OP 1 + 2
TWOGK-OP-1
Is This A Zombie?-OP 2
.hack//roots-OP
Clannad OP 1
many more, but these are from the top of my head

It's a creator's dream sequence; the theme of the graphics go perfectly with the music. That type of cohesiveness is hard to accomplish.

Agree. Anis Shimada has such a beautiful voice, too.

Ergo Proxy and Deadman Wonderland are the only openings I've never skipped.

Also like:
Saiyuki (both openings, but especially the first w/ "For Real" by Tokuyuma Hidenori; too bad I can't find anywhere to download that song, or the one from Deadman Wonderland.)
Samurai Champloo
Baccano